# [TCML] A couple of questions

futuret at aol.com futuret at aol.com
Sun Jan 10 14:03:25 MST 2010

``` Hi Rick,

If you tell me the motor HP rating and current draw, I can tell you a rough idea
of the correct capacitor value.  But there's no known way to calculate the
exact value needed.  It has to be determined by trial and error.  What I do
is take an arbitrary cap that i suspect might be about right, and install it
into the circuit.  Then I run the motor and have a voltmeter connected directly
across the motor terminals.  Then i rotate the variac knob while watching
the meter.  If the meter voltage shoots up over 135 volts, then the capacitor is
too large.  The voltage will not be highest at the far end of the variac knob  rotation,
but will be highest at some mid-way area of knob  rotation.  If there is no voltage
rise across the motor as i turn up the variac, then the capacitor is too small.
I like to see a 5 to 10 volt rise over the 120V input.  So the meter should
read between 125 to 130 volts or so at it's highest peak, as the variac knob
is turned.  It's possible that the motor might lose sync-lock  before you
reach the far end of the variac.  This can be fixed by using a slightly larger
capacitor, while still keeping the max voltage below 135 volts or so.  lf it's
impossible to keep sync lock over the entire variac range, then a stop can
be installed on the variac to prevent the knob from being turned past that
point.  However the variac will still provide the full phase range.  This phase
range  is always limited to 90 degrees electrical.  If you have an 1800 rpm motor,
then the mechanical phase range is limited to 45 degrees, but still 90 degrees
electrical.  It may be slightly less than 90 degrees.  So when using the phase
shifter, it's still necessary to rotate the motor in the cradle (or shift the rotor
position relative to the shaft), to get the phase in the ballpark.  This is
especially true for 120 bps.    When I made my original circuit design, Terry
then suggested that I add the fuse and resistor as shown below.  For the
1/20 HP motor I used, a 47uF cap was good.  For the small oriental motors that
some folks use, only about a 4uF cap is needed.  For a large 1/4HP motor, a
200 uF cap may be needed.  The cap has to be a motor run type cap, not a motor
start cap.  It can't be an electrolytic cap.  A power factor correction type cap works
well also.  The type of caps found in old window air conditioners works well, etc.

At times I've used a cap that was a little small such that there was no actual voltage rise
across the motor.  I think one fellow said he left the cap out completely and the circuit
still worked??  The proper value for the fuse also depends on the motor HP rating and
current draw.

I looked at the website for the transformers.  They appear to be very rugged and high
quality from how they describe it.  Other than that I don't know anything about them.
I would guess they're not cheap.  My guess is that old NST's will still be around for many
years though.  Who knows when they'll become really rare?

Regards,
John

-----Original Message-----
From: stork3264 at comcast.net
To: Tesla Coil Mailing List <tesla at pupman.com>
Sent: Sun, Jan 10, 2010 1:59 pm
Subject: [TCML] A couple of questions

Hi John,

A couple of questions.  I have a synchronous SG and want to build your variac
phase changer.  I understand I need a quick blow line fuse and a motor run
capacitor.  How do I select the right capacitor rating?  Also could you send

Second,  Plasma Technics, Inc sells transformers.  They are in the ozone
producing business.  Some are current limited and their windings are suppost to
be well insulated and almost  "Bullet Proof".  I realize ordinary NSTs will not
be available in the future.  Would give your take on smaller (less than 1 KW)
120 VAC transformers.  Are they suitable for TC use?  If the price is reasonable
I may buy a smaller to mid sized unit.

Thanks,

Rick
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